1. Field of the Invention
The present invention relates to a photoconductor coupled liquid crystal light valve and a method for manufacturing the same. More particularly it relates to a photoconductor coupled liquid crystal light valve using the photoconductive effect of a photoconductive material and the electro-optical effect of a liquid crystal and to a method for manufacturing the same.
2. Description of the Related Art
FIG. 4 shows a general construction of liquid crystal light valves. Liquid crystal light valves comprise a photoconductive layer 10, a liquid crystal layer 2, a pair of transparent electrodes 3 disposed by sandwiching therebetween the photoconductive layer 10 and the liquid crystal layer 2, means 4 for applying voltage including said transparent electrodes 3, a reflecting layer 5 and an orientation treatment film 6 disposed between the photoconductive layer 10 and the liquid crystal layer 2. On the external surfaces of the transparent electrodes 3 are disposed a pair of glass substrates 7.
A basic behavior of liquid crystal light valve devices is illustrated hereinbelow. For the sake of explanation, it is assumed that the resistance value of the reflecting layer 5 and the orientation treatment layer 6 is considerably smaller than that of the photoconductive layer 10 and the liquid crystal layer 2.
Applying voltage V.sub.o to the photoconductive layer 10 and the liquid crystal layer 2 from means 4 for applying voltage energizes the photoconductive layer 10 and the liquid crystal layer 2 at a voltage corresponding to the resistance value of respective layers. When the photoconductive layer 10 is not irradiated with light, the resistance applied to the photoconductive layer 10 exhibits a large value to result in a low voltage applied to the liquid crystal layer 2, which is even lower than a threshold voltage. V.sub.th generating an electro-optical effect.
Then, applying a writing light 8 to the photoconductive layer 10 in a pattern formed by introducing or shielding light coming from the side of the photoconductive layer 10 reduces a resistance at a portion of the photoconductive layer 10 irradiated with light 9 thereby increasing voltage applied to a portion of the liquid crystal layer 2 irradiated with light coming from the side of the photoconductive layer 10. The voltage at this point becomes larger than the threshold voltage V.sub.th, an electro-optical effect generates in the liquid crystal layer 2, thereby allowing writing an image in the liquid crystal layer 2. This written image is fetched as an image 19 constituted by the absence or the presence of reflected light art he reflecting layer 5 by applying a reading light 9 from the side of the liquid crystal layer 2.
A light valve operating in the above way must satisfy the following formula to store a favorable image with a clear contrast. In the formula, R.sub.PCL designates a resistance value of the photoconductive layer in a region irradiated with light, R.sub.LC a resistance value of the liquid crystal, R.sub.PCD a resistance value of the photoconductive layer in a region not irradiated with light; EQU R.sub.PCL &lt;&lt;R.sub.LC &lt;&lt;R.sub.PCD ( 1)
Assuming that the photoconductive layer 10 (or 1 in FIG. 1) and the liquid crystal layer 2 has approximately the same thickness, the formula-can be represented by replacing resistance values with resistivity values. In other word, the following formula can be produced; EQU P.sub.PCL &lt;&lt;P.sub.LC &lt;&lt;P.sub.PCD ( 2)
Considering the properties of the photoconductive layer, increasing P.sub.PCD will increase the value of P.sub.PCD /P.sub.PCL.
As a method for attaining the object, the following two methods (a) and (b) can be considered.
(a) Using an i type film having a large band gap as a blocking layer and further overlaying an i type light absorbing layer, having a smaller band gap than the former i type film, allows the formation of an i--i type hetero junction, thereby enlarging an actual value of P.sub.PCD.
(b) Or a III group element such as boron (or V group element such as phosphorus) is doped into a film having a wide band gap to form a blocking layer. Subsequently an i-type light absorbing layer having a narrower band gap than that of the blocking layer is overlaid to provide a p-i junction (or n-i junction) with the photoconductive layer to which a reverse bias is applied to enlarge an effective P.sub.PCD.
In other words, properties required of a material which forms the blocking layer include a wide band gap which inhibits the absorption of writing light. When the band gap of the blocking layer is small, the writing light is absorbed by the blocking layer before it is absorbed by the light absorbing layer with a result that the photosensitivity deteriorates. To overcome such a drawback, an alloy type amorphous silicon such as a-SiC, a-SiN and a-SiO which has a wide band gap as a material for a blocking layer is used.
Preparing an alloy film made of a-SiC, a-SiN and the like by plasma CVD method (hereinafter referred to as PCVD method) or sputtering method requires as a material gas a mixture such as SiH.sub.4 +CH.sub.4 and SiH.sub.4 +NH.sub.3 to complicate the preparation process, which constitutes a factor of increasing the cost.
Besides, the process of laminating the alloy type blocking layer with the non-alloy type-light absorbing layer suffers from a drawback that such process will cause the contamination of carbon, nitrogen and oxygen atoms in the blocking layer into the light absorbing layer, thereby failing in giving desired properties.
On the other hand, Japanese Laid-Open Patent No. SHO 58-34435/1983 Publication discloses a three-layered photoconductor coupled liquid crystal light valve having a photoconductive layer formed with PCVD method. As an embodiment thereof, the following three methods are disclosed;
(1) a method for forming a p-i-n structure consisting of an i-layer formed by using a-Si:H film, a p-layer and an n-layer provided on both side of said. i-layer by doping phosphorus and boron respectively therein.
(2) a method for forming a three-layer structure consisting an i-layer using an a-Si:H film, an alloy type blocking layer having a wide band gap such as a-Si:C:H film and a-Si:N:H film provided on the side of the transparent electrodes, and an n-layer doped with phosphorus and the like and provided on the opposite side of the transparent electrodes.
(3) a method for forming a three-layer structure consisting an i-layer using an a-Si:H layer, a thin metal film formed of platinum, gold and molybdenum and the like provided on the side of the transparent electrodes, and an n-layer doped with phosphorus and the like and provided on the opposite side of the transparent electrodes.
However, the above Publication does not disclose any method for overcoming drawbacks such as a complicated process for manufacturing the above liquid crystal light valve. The PCVD method used in forming a photoconductive layer inevitably results in producing (SiH.sub.2) powders in the deposition chamber. The publication does not disclose any method for overcoming a drawback that defects are produced in formed films owing to the deposition of (SiH.sub.2) powders on the substrate of formed films.
Besides, Japanese Laid-Open Patent Publication No. HEI 3-126920/1991, HAYAKAWA/SHARP, and corresponding U.S. Pat. No. 5,239,397 discloses a photoconductor coupled liquid crystal light valves having a photoconductive layer formed with ECR method. Although the Publication discloses an improvement in inhibiting the production of (SiH.sub.2).sub.n powders in the deposition chamber, it does not disclose the provision of a blocking layer for storing an image with a favorable contrast.